It has been demonstrated that voluntary control of accommodation may be acquired by experimental emmetropic subjects under laboratory conditions through the use of biofeedback techniques. This has been reported by T. N. Cornsweet and H. D. Crane in their Research Note entitled "Training the Visual Accommodation System" published in VISION RES., Vol. 13, pp. 713-715, Pergamon Press 1973, as printed in Great Britain. Similar results have also been reported by R. J. Randle in "Volitional Control of Visual Accommodation", Advisory Group for Aerospace Res. and Dev. (AGARD), CONF. PROC. 82. Carmisch-Partenkirchen, Germany, 15-17 Sept. 1970.
This is perhaps not surprising when analyzed in retrospect, because the focusing or ciliary muscle which is attached by zonules to the crystalline lens of the eye and controls the thickness and thinness thereof to achieve focus, is a sphincter or ring muscle of approximately 2 millimeters located directly behind the iris. The ciliary muscle contracts to make the lens thicken when something close is viewed and this muscle dilates to make the lens thinner when objects are viewed at a distance. The ciliary muscle is also controlled by that part of the nervous system that controls heart rate, body temperature, breathing, blood pressure, stomach acidity and bowel motility, and under conditions of stress the ciliary muscle will undergo spasm causing a myopic condition. Thus, given the normal vision and functions of the ciliary muscle in focusing an image on the retina in an experimental emmetropic subject and the provision of biofeedback indicative of the state of the experimental subject's accommodation, it is reasonable to expect that voluntary control of the ciliary muscle, and hence, of accommodation may be learned in a manner similar to the control of other functions governed by this portion of the nervous system as obtained, for example, through disciplines such as transcendental meditation, yoga and the like.
The application of voluntary control of accommodation to reduce limited functional myopia has also been the subject of experimentation as reported in "Biofeedback of Accommodation to Reduce Functional Myopia: A Case Report" by Joseph N. Trachtman in THE AMERICAN JOURNAL OF OPTOMETRY & PHYSIOLOGICAL OPTICS, Vol. 55, No. 6, pp. 400-406, June 1978; and in "Biofeedback of Accommodation to Reduce Functional Myopia" by Joseph N. Trachtman, Vincent Giambalvo and Jerome Feldman, BIOFEEDBACK AND SELF-REGULATION, Vol. 6, No. 4, 1981. Here functional myopia was defined as being limited to -1.25 diopters, and while improvements in the relatively slight myopic conditions of three experimental subjects were demonstrated, the same were somewhat limited.
In the research reported by T. N. Cornsweet and H. D. Crane, for example, the experimental subjects were established within an experimental environment wherein a dim point was viewed through an artificial pupil and accommodation was measured through an automatic infrared optometer. Feedback pertaining to their state of accommodation was provided through one earpiece of a binaural headphone set while a manually alterable tone was provided through the other earpiece. The experimental subjects were, in effect, told to manually vary the tone in one earpiece and alter their accommodation on an ad lib basis until the response tone fed back matched the tone which had been set. Two experimental emmetropic subjects were utilized and each required three hours of ad lib practice to fully perform the task assigned. However, once voluntary control of accommodation was learned employing auditory tone matching techniques, the voluntary control learned was apparently retained when the feedback technique was changed to employ optical matching techniques. This, however, was purely an experiment pertaining to voluntary control of the ciliary muscle since experimental emmetropic subjects can, by definition, contract and relax their ciliary muscle to achieve proper focus.
In my own experiments, as noted above, the experimental subject's accommodation was again measured by an automatic infrared optometer. The research utilized a sophisticated experimental design (single subject, double reversal, and multiple baseline), and functionally myopic experimental subjects having a refractive error of from -0.25 to -1.25 diopters were employed. The training was conducted in a dark environment and testing was conducted under computer control in such manner that baseline and feedback periods were interleaved according to a single subject, double reversal, multiple baseline technique which allows the experimental subject to act as his own control. More particularly, during each baseline period a green fixation dot placed beyond optical infinity was actuated and the experimental subject was requested to look at the light and depress a response key. This continued for eight seconds and then the fixation light was deenergized and the experimental subject allowed to rest his eyes for four seconds only. White noise was supplied to the headphones during the interval when the fixation dot was illuminated and each minute of baseline measurement by the optometer operating under computer control was made up of five cycles, as outlined above. Baseline periods made up of one minute or more were then followed by training periods randomly chosen of unequal duration of the same makeup as the baseline intervals, i.e., eight seconds of training followed by a four second rest interval.
During training periods, feedback in the form of tone information was provided to the headphones under conditions where the tone produced was directly proportional to the accommodative status of the experimental subject, each 0.125 diopter change in accommodation producing a 50 Hz change in tone. For purposes of providing feedback tone information, the mean of two consecutive optometer scans was calculated by the computer, wherein one scan occurred each 31.6 milliseconds and a tone proportional to the mean calculated was provided to the experimental subject within 134 milliseconds of the onset of the accommodative response. The experiments indicated that a clinically significant 0.5 diopter reduction of myopia could be achieved for myopic experimental subjects in the 0.75 to 1.25 diopter range. This corresponds to a Snellen visual acuity change of from 20/65 to 20/25.
Here, too, the experimental subject was merely instructed to make the tone fed back during training intervals as high as possible. The experimental subject was then left alone and the experimental procedure was automatically conducted by the computer. In the experiments conducted, experimental subject fatigue proved to be a problem due to the fixed number of baseline and feedback periods employed and the fixed number of sessions to which each experimental subject was obligated to experience. This periodically resulted in acute spasm of the ciliary muscle manifesting itself as a burning sensation in the eye of the experimental subject. It was also necessary to dilate each experimental subject's pupils to facilitate the measurements to be taken, and it was found that the delay of 134 milliseconds between the initial accommodative measurement and the delivery of the tone by the computer, due to the calculations employed, was too long. Additionally, the computer was employed to calculate the relationship between diopters and the output of the optometer using a least squares fit, and such calibration was performed for each experimental subject for each experimental training session. Alignment of the experimental subject with the instrument was performed solely by observing the output from the retinal image on an oscilloscope, and this technique also proved to be time consuming.
In the experiments conducted all of the training was performed in a dark room in order to achieve a break in the accommodative convergence reflex and to eliminate as many stimuli to accommodation as possible. Thus, the experiments were performed for the purpose of demonstrating that control of accommodation in functionally myopic experimental subjects could be learned in the absence of all stimuli other than voluntary control. There was no attempt to generalize the voluntary control of accommodation which was mastered to an environment which contained stimuli or to permit the same to be implemented in an environment which also contained blur cues. Finally, in the experiment conducted a Badal lens system, having movable Snellen letters mounted on a track, was employed for purposes of initially calibrating the optometer for each training period and a least squares fit was tested to insure a linear monotonic calibration was achieved.
U.S. Pat. Nos. 4,533,221 and (Ser. No. 829,555), of which the instant application is a continuation-in-part, are directed to methods and apparatus for performing accommodation training under clinical conditions to teach voluntary control of accommodation and reduce various visual accuity problems in a patient. Accommodation has been defined as the process of increasing and decreasing the refractive power of the crystalline lens of the eye. For clear distance viewing, a decrease in refractive power obtained through a relaxatin of the ciliary muscle is required; while for clear near viewing, an increase in refractive power is mandated. Functional myopia, as distinguished from congenital or pathological myopia, has been defined as myopia due to a spasm of the ciliary muscle. Conversely, absolute hyperopia is related to an inability to contract the ciliary muscle to increase the refractive power of the lens. Hence, voluntary control of accommodation in a patient will allow a marked increase in the ability of a patent to decrease or increase the refractive power of the lens of the eye, and hence, markedly reduce any marked myopic or hyperopic condition associated therewith.
In the case of functional myopia, for example, simple acquistion of voluntary control of accommodation in a patient has been found insufficient to enable that patient to correct for a blurred image received unless the ability to extend such voluntary control over accommodation is generalized to a normal environment to enable the patient to voluntarily control accommodation in the presence of blur cues and the accommodative convergence reflex. More particularly, my clinical experience with functionally myopic patients has indicated that a general response to perceiving a blurred image is for the patient to overfocus to factually increase blur. This is readily demonstratable when patients are reading eye charts after undergoing some accommodation training. So long as there is something on the eye chart which can be read, voluntary control will properly operate to enable the patient to relax focus, and hence, read succeeding lines on the eye chart. However, when the chart is changed and difficulty is encountered with the first line of the chart, the patient frequently will cause the entire new slide to be blurred out. Instead, if a new chart is provided with at least one large letter which can be easily read, the patient's progress on the next slide will continue in a normal manner wherein the patient may relax his focus and continue reading succeeding lines on the chart. Thus, once a patient has acquired the skill of voluntarily controlling accommodation in an environment free of all blur cues, this control must be extended or generalized to an environment which contains such blur cues.
Similarly, for voluntary control of accommodation to be useful to a patient in controlling focus, that patient must be able to control accommodation in the presence of the neurological reflex known as the accommodation convergence reflex. This reflex, in effect, controls the movement of the eyes toward and away from each other as a function of the accommodation of the eye. Thus, as accommodation is increased, the accommodation convergence reflex will cause the eyes to turn toward one another; while when focus or accommodation is relaxed, the eyes will assume a more parallel relationship so that at optical infinity, an emmetropic patient will have zero accommodation and zero convergence. Accordingly, for a patient to effectively use voluntary control of accommodation to improve vision, such voluntary control is best learned in an environment free of the accommodation convergence reflex and thereafter must be extended to an environment wherein the accommodation convergence reflex is present so that learned voluntary control of accommodation is operative in the presence of this neurological reflex.
Finally, research has indicated that the accommodation mechanism of the eye is subject to 2,047 possible combinations of stimuli which can also trigger the accommodation mechanism. Hence, for any learned voluntary control of accommodation to be useful to a patient in reducing or curing a visual acuity problem, the voluntary control of accommodation which is acquired must be fully operative in the presence of such stimuli.
While each of the considerations noted above are applicable to myopic patients, corresponding considerations also apply to hyperopic patients. For these reasons, the methods and apparatus for performing accommodation training under clinical conditions for purposes of teaching voluntary control of accommodation as taught herein, initially establish an environment wherein a patient is able to acquire control of accommodation free of all blur cues and stimuli to accommodation and where the accommodation convergence reflex loop is broken. Under these circumstances, it will be appreciated that the patient need only learn to control accommodation through the action of the ciliary muscle through positive inputs to the autonomic nervous system. Under these circumstances, biofeedback is provided for purposes of enabling the patient to gauge which of such inputs are achieving the desired result, as well as the degree of control of accommodation which is being exercised. This is continued until such time as the patient acquires substantial voluntary control over accommodation and can promptly control the state of accommodation or relaxation associated with the ciliary muscles to desired levels.
Once proficiency in controlling accommodation is demonstrated absent blur cues, accommodation stimuli, and absent the accommodation convergence reflex, the training is generalized to provide an environment wherein the voluntary control over accommodation may be exercised in the presence of these factors. This, however, is slowly done under circumstances where blur cues of graduated character are inserted, or alternatively, stimuli to accommodation and the accommodative convergence reflex are gradually provided so that the patient's ability to voluntarily control convergence is retained as the environment is slowly generalized. When the patient is able to maintain voluntary control over convergence in an environment wherein the accommodative convergence loop and stimuli to the accommodation mechanism are fully restored and where blur cues of various character are present, the voluntary control of accommodation which has been learned is fully usable by the patient to reduce various visual acuity problems. While the methods and apparatus for performing accommodation training under clinical conditions as taught herein have obvious application to myopic and hyperopic patients, the same have been also found to be useful in treating presbyopia, anisometropia, nystagmus, and strabismus and eccentric fixation.